Total Wars for Victoria 2
After no small amount of discussion in this thread regarding whether or not it would be possible to simulate the "Great War" in Victoria 2-- considering all the variables that needed to be in place for World War I which very likely would not be repeated in your average game-- I had an idea of how it might work. So the goal was to allow great powers to have late-game wars that would draw in their allies result in severe effects for a defeated country without having an overly deterministic setup-- and, after a great deal of tinkering, got it implemented.
NOTE: Since the "Great War" system was added in AHD, this has been renamed to "Total Wars". The vanilla Great Wars still occur when any war has more than one great power on each side, and makes wargoals cheaper-- the Total War system exists on top of that. A Great War could involve this system, if the Dismantle CB is invoked, but need not.
DOWNLOAD
For "Heart of Darkness", v3.01: Great Wars 1.9
For "A House Divided", beta patch 2.3: Great Wars 1.8
Instructions for making Great War compatible with other mods are included in the text file.
More detailed info on the Great War system can be found on the
PDM Wiki.
What It Does
* As soon as any great power invents Mass Politics, they will have a "Great War" casus belli show up on any other great power with whom they have -150 relations or less (and who is not already disarmed). This CB will occasionally be granted by event (and, in AHD, can be constructed). If you are already at war with another country, the CB costs 0 infamy to invoke.
* A "great war" country modifier which reduces war exhaustion so winning countries are far less likely to peace out, but still allows war exhaustion to build in the loser.
* Once a great power is in a Great War, they will be prompted to seek out allies from those countries who are hostile to their enemy, who have cores in the enemy country or who are ideologically compatible. This creates an alliance and increases relations.
* After the above, the country calls all its allies into the existing war. Note that if you pay attention to the event ticker, you'll occasionally see something like "The Russian Empire makes peace with ." -- that's just the hack by which the Call Ally function is available, since there's no command to call individual countries by command. Just ignore it.
* Allies who are also greater powers will also call THEIR allies.
* While in a great war, extra casus bellis will appear: they are 0-infamy and lower-warscore versions of Free Peoples, Take From Sphere, Release Vassal, Place in the Sun and Acquire Core. You can only use them on nations you're already at war with, and the AI will use them as well. (In AHD, there is an additional "Install Government" CB which becomes available if their capital province is occupied.)
* When the Great War is done, if any greater or secondary power involved is disarmed (meaning a Cut Down to Size or Great War casus belli has successfully been enforced), the country's empire will be dismantled. This means non-existing countries in their borders will be released, cores returned, many of their colonies lost, etc... and they'll also experience a rise in fascist sentiment, not to mention a "national humiliation" modifier. (In AHD, countries are only disarmed if the Great War CB is enforced.)
* Specific dismantling events have been written for the most common great powers, so Germany won't release all its German minors, England releases Ireland and its colonies but not Scotland, etc.
What It Does Not Do
* This is NOT a way to create World War 1 in Victoria 2. All it does is make the late wars in the game potentially far more threatening than they currently are... a true "great war" can result if there are enough nations involved in it and conditions are right, but it may simply be a war like many others. The differences are that allies will always be called (which does not happen normally) and a country can seek out and obtain allies even while they're at war (again, something that does not happen normally).
* The "dismantling" effect of a great war only occurs if one of the countries involved ends the war disarmed. That requires a considerable defeat. In many cases the great war might simply mean a war where many wargoals are involved (since they are much less "expensive", the AI will invoke them more often). (As above, in AHD the dismantling effect only occurs if the Great War CB is enforced.)
* While allies are sought out, there's no way to force them to join the war. It depends entirely on the AI's choice. So while there's a greater chance of getting more allies in the action, we can't actually have factions like in the actual World War. There's a chance something like that might occur, but it's in no way guaranteed.
This isn't mean to underplay what the mod does, simply to set expectations appropriately.
The war begins!
Shall we call our allies?
Peace at last!
The Treaty of Vienna is signed!
The results of the treaty:

NOTE: The dates on the above screenshots don't reflect how this will work in the game-- events were fired manually for screenshot purposes. Since Mass Politics cannot be invented until after 1900, Great Wars can only begin after that point.
WARNING: There is a bug in the non-AHD version of the game (which cannot be modded) that affects this mod. If you enforce a Great War CB or Cut Down to Size CB on a country while you're still at war (with anyone), they will pay you reparations but they WILL NOT BE DISARMED. This means the dismantling event will not fire. Just be warned.
Any comments or suggestions for changes will be welcome. This work is included in the PDM mod.
UPDATED with v1.9:- Compatibility with Heart of Darkness expansion pack, v3.01.
- Added CB's to install Fascism & Communism, as well as install Democracy on Fascist/Communist countries, outside of a Great War
UPDATED with v1.8:- Compatibility with A House Divided expansion pack, beta patch 2.3.
- The Great War CB has been renamed to "Dismantle Nation" -- it works independently of the Great War functionality which was added in the 2.3 patch. So wars which invoke the Dismantle CB can cause Great Wars if enough nations get involved, but are not Great Wars in and of themselves. Invoking the CB will start the country into "total war" mode... which otherwise works exactly as the system does now.
- The mod directory is now used.
UPDATED with v1.7:- Compatibility with A House Divided expansion pack (1.7 will only work with the expansion), beta patch 2.21 or earlier.
- The Great War system now only kicks in when a Great War CB is invoked.
- Dismantling of an empire now only occurs when the Great War CB is enforced. The treaty event will always fire, regardless of whether the country is disarmed.
- A new "Install Government" CB is available during a Great War, requiring that the enemy's capital province be occupied before it can be invoked. It may be used by democracies/hms governments on either fascist or communist countries, by fascists on non-fascist countries or by communists on non-communist countries. If enforced, it changes the target's government type as appropriate, makes the target friendly to you, sets up an alliance and will give you some diplomatic influence if they're not a GP.
- A dismantling event specific to the Ottoman Empire has been added.
- A fix to prevent the disbursement of colonial provinces from stalling has been added.
- The country-specific dismantling events will now occur regardless of whether or not they are still GP's or SP's.
UPDATED with v1.6:- Compatibility with the Sept 9th beta patch (1.6 will only work with the 1.4 patch).
- Added two events that can fire after Mass Politics is invented. These events grant a Great War CB against enemy countries you are currently at peace with. Prior to getting these events, starting a war with the Great War CB costs infamy-- you are still able to apply the Great War CB to any country you are currently at war with.
- Relation levels with a dismantled country and any other country increased. The dismantled country also has its infamy set at 24.99.
- Uncivs are no longer released as a result of dismantling.
UPDATED with v1.5:- Added the Infamy War events. Now, when a country gets too high in Infamy, it will be warned by the strongest AI great power existing to dismantle... and should the Infamy country refuse, that country will begin a Great War against it even if Great Wars have not been activated yet. Other countries will also be drawn in against the Infamy country, forming a coalition of sorts.
- The dismantling events no longer check for whether overseas or colonies are cores.
- Updating AoC compatibility to AoC 3.2 using the separate download, and also made Great War inherently compatible with NNM 1.6.
UPDATED with v1.4:- Revamped the colonial assignation system completely. It will spam events a bit, as an event is required for each colonial state, but this way all the colonies and overseas states of a defeated power will get assigned to a victor-- and hopefully not end with borders that are too crazy.
- Any power that has had 75% of its territories occupied for a month can end up getting the treaty event (so being disarmed may not be required).
- Austria-specific dismantling event included.
- Any country that's involved in a great war will be heavily restricted on declaring war for 3 years. Any country that's been dismantled will be unable to declare war at all for 10 years. This did involve larger changes to the cb_types file, which makes it more difficult to combine the mod with other mods... so I've included a version of Great War that is specifically for AoC 3.1, and instructions on making it compatible with some other popular mods. PDM, of course, does not require this.
UPDATED with v1.3:- All non-allied neutral countries receive cores back, but they get theirs last after everything else is figured out.
- Non-existing countries are only released so long as there is at least one core province which isn't colonial.
- Colonial disbursement has been revamped, giving only states that are coastal and/or neighboring to your territories, and then possibly any neighboring states to THAT. It won't give colonies that are deeply interior, but at least it now won't give isolated land-locked states that the country can't possibly access.
- a player victor gets "first dibs" on colonies, meaning the AI events that take colonies don't fire until after the player has made his selection (otherwise the AI events fire at the same time, and are therefore "faster" and get their provinces first).
- The Great War CB now correctly requires -100 relations or less (not more).
- Someone who ends a Great War not disarmed only gets "no more war" for 3 years, not 10.
- The MTTH for the events which bring on the reduced country modifiers has been shortened.
UPDATED with v1.2:- Made all colony/overseas territories dole out by state only, to prevent messy borders (particularly in Africa).
- Colonies in Africa more likely to go to a GP only.
UPDATED with v1.1:- There are now three country modifiers instead of one, with the others replacing the first with a speed variable on how well the country is doing.
- Expanded the list of approached countries a bit more.
- The colonial release script looks for overseas OR colonial provinces, not provinces that must be both.
- Released countries now won't perform secondary releases on countries of the same culture group as the treaty country.
- The "War is Upon Us" event now instructs a player how to dismantle their opponent.